Conventional resistance spot welding techniques employ a method by which metal surfaces are joined together in one or more spots. Workpieces are held together under force by one or more electrodes. The contacting surfaces are heated by a pulse of high amperage current generated by contact with the electrodes to form what is known as a weld nugget at the interface between the surfaces. When the flow of current ceases, the electrode force is ordinarily maintained for a short period of time to allow the weld nugget to cool and solidify forming a strong mechanical bond. An excellent discussion of the details of the metallurgical phenomena that occurs during resistant spot welding is found in Nied "The Finite Element Modeling of the Resistance Spot Welding Process", Welding Research Supplement, pp. 123-132 (Apr. 1984).
A multi-gun resistance welding apparatus comprises a plurality of resistance welding guns for simultaneously performing many resistance welds. The multi-gun apparatus can be in the form of a press in which the plurality of guns are mounted, or in the form of individually positionable guns which can advance upon a workpiece, perform a weld, and then retract from the workpiece. The guns can be advanced and retracted either manually, or robotically using an automatic indexing device. The use of multi-gun resistance welding machines is popular for high-volume production work such as joining automotive components for both passenger and commercial vehicles. Typical applications include complete truck cabs, air vent cowls, air cleaner components, passenger car hoods, motor compartments, body sides, rear quarters, deck lids, and load floors. The popularity of multi-gun resistance spot welding is due in large part to its capability of rapidly producing welds with an apparatus that can be used in automated production.
A brief description of a representative task performed by a multi-gun welding system is as follows. First, a workpiece which is to be welded using the multi-gun welding system is positioned by means of a weld part fixture. For the purpose of the present invention, the term "workpiece" can denote a plurality of workpieces which are to be welded together, or in a case such as projection welding, can denote a single workpiece onto which another workpiece loaded in the gun is welded. After the workpiece is positioned, a plurality of weld guns (not necessarily all of the weld guns of the system) advance upon the workpiece. Each of the weld guns comprises at least one electrode, wherein each electrode is positionable with respect to its corresponding weld gun by a powered cylinder along a cylinder rod axis. A common configuration for electrodes within a weld gun has two electrodes, one being a welding electrode and the other being a ground electrode, in opposing relation to each other. A configuration for single-electrode resistance welding has the ground electrode permanently attached to the top of the part fixture.
A plurality of the electrodes then advance upon the workpiece, causing electrical contact to be initiated with the workpiece. Next, a voltage potential is applied to each of the plurality of electrodes. This causes an electrical current to flow through the electrodes (welding and ground), whereby a resulting electrical heating induces the formation of weld nuggets. Finally, the electrodes are retracted from the workpiece. The workpiece can then be translated or rotated so that another plurality of electrodes can be advanced upon the workpiece for further welding, or another workpiece can be introduced to the apparatus for welding.
Although multi-gun resistance spot welding has many advantages for high-volume production work, there are some complications that can occur. First, the positioning of the workpieces before welding power is applied, known as pre-weld fit-up, significantly affects weld quality and electrode life. A poor fit-up condition can occur when workpieces of superior quality are improperly positioned with respect to one another. A poor fit-up condition can also occur with properly positioned workpieces of inferior quality. A further condition of poor fit-up can occur when superior-quality workpieces, positioned properly with respect to one another, are not positioned properly with respect to the electrodes. Poor fit-up conditions reduce effective weld pressure by an amount required to squeeze the workpieces together. This reduction of pressure can lead to excessive weld heat, and, more severely, possible weld blowouts which increase electrode wear and further can require an electrode change. Moreover, improper position and orientation of the workpieces with respect to the electrodes can produce inferior quality welds.
U.S. Pat. No. 5,220,145 to Cecil et al. discloses a single-gun welding control system that monitors electrode displacement for detection of the poor fit-up condition. This system can detect poor fit-up caused by workpieces improperly positioned with respect to one another and by inferior-quality workpieces. However, it cannot spatially gauge the position and orientation of the workpieces due to its application in a single-gun apparatus.
A second difficulty in multi-gun resistance welding is in controlling the weld process satisfactorily in order to produce consistently good welds. Many different factors must be controlled such as voltage, current, pressure, heat loss, shunting, water temperature, and electrode wear, as well as the thickness and composition of the workpiece material. Many of these variables are difficult to consistently control. Several attempts have been made to automatically control resistance spot welding processes. For example, some techniques have been designed to regulate the amount of energy used during the weld cycle. To this end, current sensors and voltage regulators have been incorporated into feedback systems to compare the detected levels with certain preset references. These feedback systems are disadvantageous from the standpoint that they do not directly detect physical characteristics of the weld itself but instead rely upon detection of secondary parameters. This can lead to poor weld quality when uncontrolled parameters vary from nominal operating conditions.
Other techniques provide means for determining whether the metal of the workpieces have reached a molten state. If the metals to be welded do not reach the temperature required to become molten, an insufficient weld could result. It has been shown through measurements that when the molten state is reached, the electrodes, which are being forced against the workpiece, begin to move into the metal. Accordingly, it has been suggested that the detection of melting by sensing subsequent inward movement of the electrodes, called indentation or penetration, is a potentially good way of determining the state of the weld. However, just because the metal reaches a molten state, does not always ensure that a good weld is made. For example, too much weld current will produce melting, but will not necessarily produce the formation of the weld nugget which is an important factor in generating a good weld. Other parameters will effect the size and configuration of the weld nugget and the many prior techniques of merely sensing inward movement of the electrodes into the workpieces cannot readily determine the extent of weld nugget growth. Thus, penetration alone is insufficient to determine weld quality.
A resistance spot welding apparatus in U.S. Pat. No. 4,542,277 to Cecil discloses a device that automatically and consistently detects the quality of resistance spot welds. This device, however, requires a custom two-ended cylinder for mounting the sensor assembly. A shortcoming of this configuration is that it is not suitable for welding applications which require the space to the back of the cylinder for other functions such as the placement of mounting assemblies. Further, the two-ended cylinder configuration does not provide for electrical isolation of the sensor necessary in welding operations due to the high currents produced. Lack of electrical isolation produces noise on the sensor signal. Some of the noise can be eliminated using software processing algorithms or processing circuitry. However, this extra processing cannot be provided without cost, and at best, it will filter some of the desired signal and leave some of the noise behind.
A further obstacle in resistance welding occurs when an electrode becomes fused to a welding surface after completion of a weld, known as a stuck-gun condition. If the multi-gun welding system does not detect the stuck gun before attempting to reposition the workpiece or advancing other weld guns upon the workpiece, extensive damage to the apparatus is possible.